There are known a number of thermal imaging systems such as ink jet printing as well as thermal imaging systems which utilize thermal transfer of a colorant such as a dye from a donor sheet to a receiver material. A wide variety of specific mechanisms for thermal transfer have been used heretofore but, as described in “A New Thermal Transfer Ink Sheet for Continuous-Tone Full Color Printer”, by M. Kutami et.al., J. Imaging Science., 1990, 16, 70–74, all fall into two broad categories. In the first of these, dyes thermally diffuse from a polymeric binder on a donor sheet into a second polymeric layer on a receiving sheet, in a process commonly called “dye diffusion thermal transfer” (or D2T2; this process is also sometimes called “dye sublimation transfer”). In this type only the dye is transferred, not the binder in which the dye is dispersed on the donor sheet. The second category is commonly referred to as “thermal mass transfer” and in this type both a dye and a carrier vehicle are transferred together from the donor sheet to the receiver sheet.
Thermal mass transfer methods may be further subdivided into those involving differential adhesion, in which a heated plug of material from the donor sheet adheres to the surface of the receiving sheet, and those involving flow penetration in which the colorant on the donor sheet melts at elevated temperature and is transferred into pores on the receiver sheet.
A common problem with images formed with such thermal imaging systems is fading of colors of the images with time because of exposure to routine environmental factors to which images are typically subjected. Such factors include normal wear and abrasion, exposure to visible and ultraviolet light, exposure to moisture, contact with skin oils as a result of being handled by a person's fingers, exposure to air and exposure to ozone. Thus, there is a continuing need for protective overcoat materials which can alleviate the fading of the dyes which form the images.